This invention relates to the radiation of circularly polarized radiation from an array of radiators and, more particularly, to the inhibiting of cross polarization among neighboring cylindrical radiators in an array antenna of the radiators for improved isolation of left hand and right hand circularly polarized signals.
Communication systems frequently employ antennas for communicating over long distances. For example, the communication systems employing a satellite encircling the earth may employ a microwave electromagnetic link between the satellite and a transmitting/receiving station on the earth. In order to provide well-defined microwave beams, it is common practice to employ an antenna on the satellite with the antenna being constructed of a plurality of radiating elements, or radiators, arranged in an array. Typically, a reflector of microwave energy is positioned in front of the radiators to aid in focusing rays of radiation to provide a desired narrow beam directed to the station on the earth.
One form of radiated signal which is employed in communication systems is a circularly polarized electromagnetic signal. A single radiator can radiate simultaneously a circularly polarized wave of clock-wise or left-hand circular polarization, and a circularly polarized wave of counter clockwise or right-hand circular polarization. Preferably, the electric field of one of the waves is orthogonal, or perpendicular, to the electric field of the other wave so as to ensure that the two waves can be received separately without interfering with each other. This permits two separate signals to be transmitted at the same carrier frequency for a doubling of the data capacity of the communication link without increasing the frequency spectrum. Microwave structures for the simultaneous generation of orthogonal circularly polarized waves have been employed often in communication systems to take advantage of the increased channel capacity.
Of particular interest herein is an array antenna transmitting signals at one frequency and receiving signals at a second frequency which is higher than the transmitting frequency. The signals on transmission employ both left and right-handed circularly polarized waves, and the signals upon reception employ both left and right hand circularly polarized waves. It is of interest to provide a desired directivity pattern to the transmitted beam, as well as the received beam of microwave radiation.
As is well known, the spacing, on centers, between radiators of the array is an important parameter in establishing a desired radiation pattern. Herein, a specific radiator spacing is to be employed, namely, a spacing equal to one wavelength of the transmitted radiation. Since the received radiation is at a higher frequency, the effective radiator spacing is greater than one wavelength for the received radiation. In addition, the array under consideration herein is to employ cylindrical radiators arranged side-by-side in the array. Typically, such cylindrical radiators are configured as circular sections of thin-walled circular waveguide.
A problem arises in that the electric fields of the transverse-electric wave which is the dominant mode in the cylindrical waveguide may depart somewhat from perfect linearity across the radiating aperture of a radiator. For example, an electric field vector located at the center of the radiating aperture may be perfectly straight while electric field vectors displaced to the right and to the left of the central vector may be partially bowed. Ideally, all of the electric vectors of one circularly polarized wave at the plane of the radiating aperture should be straight, or linear, rather than bowed, and should be perpendicular to the corresponding electric field vectors of the other circularly polarized wave. However, due to the bowing of the electric field in each wave, there is a small vector component of one wave which is parallel to a small vector component of the other wave allowing for a cross-coupling of signals upon reception of the respective waves at the station on the earth or at the satellite. Such cross coupling, or cross polarization, is to be avoided as much as is possible to insure highest quality reception of signals communicated by the array antenna. The forgoing problem exists both in the case of transmission from an array of radiators as well as in he transmission from a single radiator.